1. Field of the Invention
The present invention relates to an illumination system which forms a color image using a scrolling operation, a projection system, and a method of forming a color image, and more particularly, to an illumination system which improves the optical efficiency by providing light having a Gaussian distribution in a color scrolling direction to a slit, a projection system using the illumination system, and a method of forming a color image.
2. Description of the Related Art
Projection systems are classified as 3-panel projection systems or as single-panel projection systems according to the number of light valves for controlling the on/off operation of light emitted from a high-output lamp on a pixel-by-pixel basis and forming a picture. Single-panel projection systems can have a smaller optical system than three-panel projection systems but provide an optical efficiency of ⅓ less than that of three-panel projection systems because R, G, and B colors into which white light is separated are used in a sequential method. As a result, attempts to increase the optical efficiency of single-panel projection systems have been made.
In a conventional single-panel projection system, light radiated from a white light source is separated into R, G, and B colors using a color filter, and the three colors are sequentially transmitted to a light valve. The light valve appropriately operates according to the sequence of colors received and creates images. As described above, a single-panel optical system sequentially uses colors, so the light efficiency is reduced to ⅓ of the light efficiency of a three-panel optical system. A color scrolling method has been proposed to solve this problem, wherein white light is separated into R, G, and B colors, and the R, G and B colors are simultaneously sent to different locations on a light valve. Since an image cannot be produced until all of R, G, and B colors for each pixel reach the light valve, color bars are periodically scrolled.
In a conventional single-panel scrolling projection system, as shown in FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104 and a polarized beam splitter array 105 and is separated into R, G, and B beams by first through fourth dichroic filters 109, 112, 139, and 141. In particular, the red beam R and the green beam G, for example, are transmitted by the first dichroic filter 109 and travel along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The second dichroic filter 112 transmits the red beam R along the first light path L1 and reflects the green beam G along a third light path L3.
First, second, and third prisms 114, 135, and 142 are rotatable and disposed on the first, second, and third light paths L1, L2, and L3, respectively. The light emitted from the light source 100 is separated into the red beam R, the green beam G, and the blue beam B, and they are scrolled while passing through corresponding first through third prisms 114, 135, and 142. As the first through third prisms 114, 135 and 142 rotate at a uniform speed, R, G, and B color bars are scrolled. The green beam G and the blue beam B that travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and then combined. Finally, the R, G, and B beams are combined by the fourth dichroic filter 141. The combined beam is transmitted by a polarized beam splitter 127 and forms a picture using a light valve 130.
A condensing lens 107 is disposed next to the polarized beam splitter array 105, and light path correction lenses 110, 117, 131, 137, and 145 are disposed along the first through third light paths L1, L2, and L3. Condensing lenses 120 and 140 are disposed between the first and fourth dichroic filters 109 and 141 and between the third and fourth dichroic filters 139 and 141, respectively. A focusing lens 124 and a polarizer 125 are disposed on the light path between the fourth dichroic filter 141 and the polarized beam splitter 127. Light path changers, for example, mirrors 118 and 133, are further disposed on the first and third light paths L1 and L3, respectively.
The scrolling of the R, G, and B color bars due to rotation of the first, second and third prisms 114, 135, and 142 is shown in FIG. 2. Scrolling represents the periodic movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms corresponding to R, G, and B colors are synchronously rotated. As described above, when R, G, and B color bars circulate one cycle, one frame of a color image is formed.
The light valve 130 processes an image signal for each pixel and forms a picture. The formed picture is magnified by a projecting lens (not shown), and the magnified picture is projected on a screen.
First, second, and third slits 113, 134, and 143 are installed in front of the first, second, and third prisms 114, 135, 142, respectively, and adjust a diverging angle (or etendue) of incident light. Etendue denotes an optical conservation quantity in an optical system. The widths of the color bars vary according to the widths of the first, second, and third slits 113, 134, and 143. If the slit widths decrease, the R, G, and B color bars are narrowed such that black bars K are formed between adjacent color bars as illustrated in FIG. 3A. On the other hand, if the slit widths increase, the R, G, and B color bars are enlarged such that overlapping portions P are formed between adjacent color bars as illustrated in FIG. 3B.
FIG. 3C shows the R, G, and B color bars formed on the light valve 130 and an optical intensity distribution for each of the R, G, and B color bars with respect to a color separation direction. The light emitted from the light source 100 has a Gaussian distribution. The light passed through the first and second lens arrays 102 and 104 has a square distribution, where the light is uniformly distributed in vertical and horizontal directions on a plane perpendicular to an optical axis. The light having a square distribution is separated into color beams, and the separated color beams travel along the first through third light paths L1, L2, and L3. The color beams pass through the first through third slits 113, 134, and 143, respectively, which remove rays that diverge at angles greater than an acceptable angle of an illumination system and thus contribute to a neat separation of color bars.
A distribution of light incident upon each of the slits 113, 134, and 143 and light D blocked thereby are shown in FIG. 3D. Since the light incident upon each of the first through third slits 113, 134, and 143 has a square distribution, a large amount of light D is removed by each of the slits 113, 134, and 143. As described above, the widths of the color bars is controlled using the slits 113, 134, and 143 to control the divergence angle or etendue of light having a square distribution. However, the removal of the large amount of light D by the slits 113, 134, and 143 disadvantageously affects the optical efficiency.